An electric generator transforms rotational energy into electrical energy according to generator action principles of a dynamoelectric machine. The turning torque supplied to a rotating rotor by a combustion or steam-driven turbine is converted to alternating current (AC) electricity, typically three-phase AC, in a stationary stator that surrounds the rotor. The generator is a mechanically massive and electrically complex structure, supplying output power up to 1,500 MVA at voltages up to 26 kilovolts. Electrical generators are the primary power producers in an electrical power system.
As shown in FIG. 1, a conventional electric generator 10 comprises a substantially cylindrical rotor 12 supporting axial field windings or rotor windings 13. A direct current (DC) supplied to the rotor windings 13 produces a magnetic flux field that rotates as the rotor rotates within a stationary armature or stator 14. One end 15 of the rotor 12 is drivingly coupled to a steam or gas driven turbine (not shown in FIG. 1) for providing rotational energy to turn the rotor 12. The opposing end 16 is coupled to an exciter (not shown) for supplying the direct current to the rotor windings 13. An alternating current is generated in the stationary stator windings as the rotor's magnetic flux field crosses the stator windings. Rotor rotation subjects the rotor 12 and the rotor windings 13 to radial centrifugal forces that may result in radial distortion of these generator components.
The stator 14, a shell-like structure, encloses the rotor and comprises a core 17 further comprising a plurality of thin, high-permeability circumferential slotted laminations 17A placed in a side-by-side orientation and insulated from each other to reduce eddy current losses. Stator coils are wound within the inwardly directed slots. The AC electricity induced in the stator windings by action of the rotating magnetic field of the rotor 12 flows to terminals 19 mounted on the generator frame for connection to an external electrical load. Three-phase alternating current is produced by a generator comprising three independent stator windings spaced at 120° around the stator shell. Single-phase alternating current is supplied from a stator having a single stator winding.
The rotor 12 and the stator 14 are enclosed within a frame 20. Each rotor end comprises a bearing journal (not shown) for cooperating with bearings 30 attached to the frame 20. The rotor 12 further carries a blower 32 for forcing cooling fluid through the generator elements. The cooling fluid is retained within the generator 10 by seals 34 located where the rotor ends penetrate the frame 20. The generator 10 further comprises coolers 36 receiving and cooling the cooling fluid to release the heat absorbed from the generator components. The cooling fluid is then recirculated back through the generator elements.
Generator cooling system is required to remove heat energy produced by electrical losses resulting from the large currents flowing through the generator conductors, including the direct current flowing through the rotor windings 13 and the alternating current induced in the stator coils. Additional heat sources include mechanical losses, such as windage caused by the spinning rotor, and friction at the bearings 30.
In a dynamoelectric motor (including rotary motors and linear motors) the stator windings are responsive to an external electric current that generates a stator magnetic field. Interaction of the stator field with a rotor magnetic field produces motion (rotary or linear) of the rotor. In an exemplary embodiment the rotor comprises a magnetically-permeable solid material, such as an iron-core rotor, for producing the rotor magnetic field.
Copper is the material of choice for the rotor's conductive windings in both generators and motors. Although the electrical resistance of copper is low compared to most other conductive materials, current flow through the copper conductors causes substantial rotor heating, diminishing the machine's power efficiency and requiring use of a cooling system to maintain the rotor at an appropriate operating temperature.
To increase generator output and efficiency and reduce generator size and weight, superconducting rotor windings with effectively no resistance have been developed. These winding are commonly referred to as high-temperature superconducting (HTS) windings (distinguished from low temperature superconducting materials that achieve a superconducting state at a lower temperature). It is preferred to use high-temperature superconducting materials since their cooling requirements are less severe.
Superconductivity is a phenomenon observed in several metals and ceramic materials when the material is cooled to temperatures ranging from near absolute zero (0° K or −273° C.) to a liquid nitrogen temperature of about 77° K or −196° C. At these temperatures the metal and ceramics exhibit effectively no electrical resistance to current flow. The temperature at which the material's electrical resistance is substantially zero is referred to as the material's critical temperature (Tc). The critical temperature for aluminum is about 1.19° K and for YBa2Cu3O7 (yttrium-barium-copper-oxide) is about 90° K. A high-temperature superconducting material is maintained at or below its critical temperature by cooling with either liquid helium or liquid nitrogen.
Since the superconducting materials exhibit substantially no electrical resistance when maintained at or below their critical temperature, these materials can carry a substantial electric current for a long duration with insignificant energy losses, including losses through the generation of heat.
Although the HTS rotor windings (coils) exhibit little resistance to current flow, they are sensitive to mechanical bending and tensile stresses that can cause premature degradation and winding failure (e.g., an open circuit). For example, it is necessary to form bends in the HTS rotor windings that circumscribe the core. Stresses are induced at these bends. Normal rotor torque, transient fault condition torques and transient magnetic fields induce additional stress forces in the rotor windings. Also, the HTS rotor winding may be subjected to over-speed forces during rotor balancing procedures at ambient temperature and occasional over-speed conditions at superconducting temperatures during power generation operation. These over-speed and fault conditions substantially increase the centrifugal force loads on the rotor coil windings beyond the loads experienced during normal operating conditions. These operating conditions must be considered in the design of the HTS rotor windings and their support structures.
Normal operation of an electrical generator involves literally thousands of start-up and shut-down cycles (i.e., cool-down cycles) over an operational lifetime of several years. The temperature excursions experienced during these operating cycles can lead to winding fatigue and must therefore be considered in the design of the HTS rotor windings.
To maintain the superconducting conductors at or below their critical temperature, coolant flow paths carrying coolant supplied from a cryogenic cooler are disposed adjacent or proximate the windings. Typical coolants comprise liquid helium, liquid nitrogen or liquid neon.
Maintaining the structural integrity of the superconducting rotor windings against static and dynamic loads presents a formidable challenge to the development of a high temperature superconducting generator. The HTS rotor windings must be adequately supported by a winding support system to withstand the forces, stresses, strains and cyclical loads of normal and fault condition generator operation described above. Moreover, the support system must ensure that the windings do not prematurely crack, fatigue or break. Finally, the coil support structure must insulate the “warm” rotor (typically operating at room temperature) from the cryogenically-cooled HTS superconducting windings to maintain the windings at or below their critical temperature.